CD59

CD59, also called protectin, is a small GPI-anchored glycoprotein of roughly 20 kDa present on nearly all cell types, including erythrocytes. It is the principal membrane inhibitor of the complement terminal pathway, binding the C8 and C9 components of the nascent membrane attack complex and preventing the polymerization of C9 into a pore. Loss of CD55 and CD59 from blood cells due to defective GPI anchor synthesis underlies the complement-mediated hemolysis of paroxysmal nocturnal hemoglobinuria. Eculizumab, a monoclonal antibody against C5, is used clinically to control this hemolysis. Beyond complement, CD59 also participates in T cell activation and insulin release.